I agree that there is no "need" for students to socialize through social networking sites; but at the same time I believe that this is simply another phenomena of the electronic age which facilitates contact between young people who do, in fact, need socialization with their peers. Socialization is one of the few ways that they will learn social skills and the ability to adapt to and feel comfortable with others. As was pointed out, this was formerly done by meeting at teenage "hangouts," talking on the phone, etc. Social networking simply makes this more convenient and easier. Parents normally wouldn't object, because teenagers are now not tieing up the phones as they did in days of yore!
There is of course, danger in social networking as there are, sadly dangerous types online who pose as someone whom they are not. For that reason, young people should be closely supervised while networking, and also be warned about "relationships" with people they only know online. There have also been instances of bullying, not always by the person whose name appeared as the communicator. However, as long as the contact is between known friends and is not harmful, it can be a quite useful tool of communication for kids.